The present application relates to a non-aqueous electrolyte secondary battery and a method for producing an electrode.
In recent years, as electronic technology has advanced, various electronic devices are improved in performance and further reduced in size and can be portable. As a result, a demand for battery used to such electronic devices, which has a higher energy density, has been increased. As a secondary battery for electronic device, aqueous electrolyte secondary batteries, such as a nickel-cadmium battery and a lead battery, have been used. These secondary batteries have a low terminal voltage at discharging and are unsuitable for obtaining a high energy density.
Recently, with respect to the secondary battery as a substitute for the nickel-cadmium battery and the like, research and development are vigorously made on a non-aqueous electrolyte secondary battery using as an anode active material capable of being doped/dedoped with/from a lithium ion, such as a carbon material, using as a cathode active material a lithium composite oxide, such as a lithium-cobalt composite oxide, and using as an electrolyte a non-aqueous electrolytic solution obtained by dissolving a lithium salt in a non-aqueous solvent.
This secondary battery has advantages in that the battery voltage is high and the self-discharge is small, making it possible to achieve high energy density. When the carbon material or lithium composite oxide is actually used as an active material, the material and the oxide are processed into powder each having an average particle size of 5 to 50 μm, and the powder and a binder are dispersed in a solvent to prepare an anode mixture slurry and a cathode mixture slurry, respectively. The resultant slurries are then applied to metallic foils as current collectors to form an anode active material layer and a cathode active material layer, respectively. A separator is disposed between the resultant negative electrode and positive electrode respectively having the anode active material layer and cathode active material layer on the current collector to separate the electrodes from each other, and they are placed in a battery can.
With respect to the non-aqueous electrolyte secondary battery described above, it should be noted that the non-aqueous electrolytic solution used in the non-aqueous electrolyte secondary battery has an electric conductivity smaller than that of an aqueous electrolytic solution by about double figures. Accordingly, for obtaining satisfactory battery performance, it would be desirable that the non-aqueous electrolyte secondary battery has a structure such that the electrolyte moves as easily as possible. For this reason, in the non-aqueous electrolyte secondary battery, a very thin separator having a thickness as small as about 10 to 50 μm is used as a separator for separating the positive electrode and the negative electrode from each other.
As described above, in the non-aqueous electrolyte secondary battery, the electrodes are individually prepared by applying a mixture slurry containing an active material in a powder form to a current collector to form an active material layer on the current collector, and then they are placed in a battery can. In this instance, before placed in the battery can, the electrode having an active material layer formed thereon is subjected to various steps, such as a step for stacking the electrodes and separator on one another, and a step for cutting the electrode into a predetermined electrode form.
The active material is fallen from the active material layer which is in contact with a guide roll or the like during running of the electrode raw sheet, and part of the fallen active material disadvantageously adheres back to the surface of the electrode, or metal fine particles caused in the step for placing the stacked electrodes in a battery can are disadvantageously mixed into the battery. The fallen active materials put on the surface of the electrode or the metal fine particles mixed into the battery have a particle size of 5 to 200 μm, which is equal to or larger than the thickness of the separator, and therefore they penetrate the separator in the battery assembled, thereby causing a physical internal short-circuiting.
For solving the situation, a method has been proposed in which a fine particle slurry is applied to at least one of the surface of the anode active material layer and the surface of the cathode active material layer and dried to form a porous protective film (see, for example, Japanese Unexamined Patent Application Publication No.H07-220759). A known porous protective film has a gas permeability of about 680 sec/100 ml.